As a catalyst for a high stereospecificity polymerization of an α-olefin, the following catalysts are known.
(1) A catalyst obtained by combining a support type solid catalyst component prepared by supporting a tetravalent titanium halide on a magnesium halide with an organoaluminum compound as a co-catalyst and an organosilicon compound as a third component (refer to JP-A 57-63310, JP-A 58-83006 and JP-A 61-78803).
(2) A catalyst obtained by combining a trivalent titanium compound-containing solid catalyst component with an organoaluminum compound as a co-catalyst and an electron donor compound as a third component, wherein the trivalent titanium compound-containing solid catalyst component is prepared by reducing a titanium compound with an organomagnesium compound in the presence of both an organosilicon compound and an ester compound to obtain a solid product, and treating the solid product with an ester compound, followed by successive treatment with a mixture of an ether compound and titanium tetrachloride or a mixture of an ether compound, titanium tetrachloride and an ester compound (refer to JP-A 7-216017).
(3) A catalyst obtained by combining a trivalent titanium compound-containing solid catalyst component with an organoaluminum compound as a co-catalyst and an electron donor compound as a third component, wherein the trivalent titanium compound-containing solid catalyst component is prepared by reducing a titanium compound with an organomagnesium compound in the presence of both an organosilicon compound and an ester compound to obtain a solid product, and treating the solid product with a mixture of an ether compound and titanium tetrachloride, followed by successive treatment with an organic acid halide compound, and further followed by successive treatment with a mixture of an ether compound and titanium tetrachloride or a mixture of an ether compound, titanium tetrachloride and an ester compound (refer to JP-A 10-212319).
However, an α-olefin polymer obtained by polymerization using any of catalysts mentioned above does not have stereospecificity of a satisfactory level, from a viewpoint of the facts that (1) a molded article having a higher rigidity is desired particularly in the field of injection molding, and (2) rigidity of an injection-molded article is increased with increase in stereospecificity of a polymer to be used.